question about the sigma level , please help

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Is the calculation of sigma level reasonable ?
what is the time limitation if the defect refer to reliability issue.
For instant , the current DPPM of one product defect 3000, can we say the sigma level is 4.2 ?
Here the DPPM is not the same as DPMO , and also in future, we may still can have the defect ( for example after 2 or more years, the defect happen at end customer side).
So my understanding is : we can not define a sigma level of one company ...
because that depends...

Please give your valuable comment .

Thanks very much .

Mark